Butadiene is an important base chemical and is used, for example, to prepare synthetic rubbers (butadiene homopolymers, styrene-butadiene-rubber or nitrile rubber) or for preparing thermoplastic terpolymers (acrylonitrile-butadiene-styrene copolymers). Butadiene is also converted to sulfolane, chloroprene and 1,4-hexamethylenediamine (via 1,4-dichlorobutene and adiponitrile). Dimerization of butadiene also allows vinylcyclohexene to be generated, which can be dehydrogenated to form styrene.
Butadiene can be prepared from saturated hydrocarbons by refining process or by thermal cracking (steam cracking) processes, in which case naphtha is typically used as the raw material. In the course of refining or steam cracking of naphtha, a mixture of methane, ethane, ethene, acetylene, propane, propene, propyne, allene, butenes, butadiene, butynes, methylallene, C4 and higher hydrocarbons are obtained.
Typical processes to recover butadiene from mixed C4 streams include extractive distillation processes, which may incorporate use of selective solvents. Examples of extractive distillation processes are found, for example, in U.S. Pat. Nos. 7,393,992, 7,482,500, 7,226,527, 4,310,388, and 7,132,038, among others.
The butadiene recovery processes typically use 3- or 4-column extractive distillation systems to separate a mixed C4 stream into three product fractions, including a lights/butane/butenes stream (Raffinate-1 product), a crude butadiene product, which may be sent to a conventional distillation system for further purification, and a concentrated C4 acetylenes stream, which may be sent to a selective hydrogenation unit or recycled to a cracker. The columns used may include a main wash column, a rectifier column, which is typically physically built separately from the wash column due to height limitations, and an afterwash column, which may be combined with the rectifier column in a divided wall column design.
To increase butadiene recovery from such processes, a conventional stand-alone pre-fractionator may be used to increase the butadiene concentration of the dilute feed by conventional distillation. Disadvantages of using conventional distillation pre-fractionation include the technical difficulty and cost associated with separating the butanes/butenes and butadiene, which have relatively low volatility.
Another proposed method to increase butadiene recovery has been to incorporate a stand-alone scrubber/stripper system to process the feed gas, concentrating the butadiene in the feed gas by removing a portion of the butanes/butenes. Disadvantages of using a scrubber/stripper to process the feed stream include equipment costs.
In addition, butadiene extraction units may be partially debottlenecked or expanded by replacing the existing trays (valve or sieve) with random packing (for example, IMTP® High Performance Random Packing available from Koch-Glitsch LP, Wichita, Kans.), or replacing the existing packing with higher efficiency packing (for example, Raschig Super-Rings available from Raschig GmbH, Ludwigshafen) in all 3 (or 4) columns in the extractive distillation area. Disadvantages of high efficiency packing include its inability to increase capacity past a certain point. For example, replacing trays with IMTP packing generally will allow a 25 to 40% increase in capacity, and replacing IMTP packing with high-capacity packing will generally allow an additional 10 to 15% increase in capacity. Also, the conventional distillation area must also be further debottlenecked or expanded to a corresponding degree.